Have you ever experienced you feeling lighter when swimming in the pool or a bucket feels lighter when drawing water from a well? If you do, buoyancy occurs during those events. In buoyancy, when immersed in water or any source of fluid, a body experiences a force from the downward direction opposite to the direction of the gravitational pull. This is why we experience a decrease in weight when such happens. Also, this may be the reason why needle sinks and plastic bottles float regardless of the weight.

Today you’ll get to know the definition, applications, purpose, causes, types, and how buoyancy is attained. You’ll also get to know the following:

• Density and related density
• Why object float or sink in water

Contents

## What is buoyancy?

Buoyancy or upthrust is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. It is also said to be the force that causes objects to float. Buoyancy occurs when the differences in pressure acting on opposite sides of an object immersed in a static fluid. It is also called buoyant force because buoyancy is the phenomenon due to buoyant force.

In a column of fluid, pressure increases with depth due to the weight of the overlying fluid. Therefore, the pressure at the bottom of a column of fluid is greater than that at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than that at the top of the object. This pressure difference results in a net upward force of the object.

Archimedes’ principle also explained that the magnitude of the force is proportional to the pressure difference, and is equivalent to the weight of the fluid that would otherwise than the liquid, the force can keep the object afloat. This is why an object’s average density is greater than that of the fluid in which it is submerged tends to sink. But if the object is less dense than the fluid, the force can keep the object afloat. This will only occur only in a non-inertial reference frame that has a gravitational field or is accelerating due to a force other than gravity. This is defining a “downward” direction.

The unit of a buoyant force is the Newton (N)

#### Watch the video to learn more on the explanation of buoyancy:

So, buoyancy can be defined as the upward force applied by the fluid on the object or the body when an object is put in or submerged in the fluid.

The center of buoyancy of an object is the center of gravity of the displaced volume of fluid. It is the point on the object where the force acts or the point where the force of buoyancy is applied. The force of buoyancy is vertical, so, the center of buoyancy is the point situated on the center of the gravity of the liquid that is being displaced by the object submerged.

Read more: The Three Newton’s laws of motion

## Applications of buoyancy

The application buoyancy is so vast since it can keep objects afloat in water is something we humans cannot do without. Buoyancy is used in submarines, hot air balloons, even fish and swimmers used the same principle. Below are some applications of buoyancy in the real world.

### Hot air balloon:

Because the atmosphere is containing air that exerts a buoyant force on any object. A balloon with hot air will float due to the buoyant force, it will descend when the balloon’s weight is higher than the buoyant force. This is because the balloon becomes stationary when the weight equals the buoyant force.

### Submarine:

Submarines make good use of the principle of buoyancy. A submarine has a large ballast tank that is used to control its position and depth from the surface of the sea. In submarines, water is allowed to enter into the ballast tank so that its weight becomes greater than the buoyant force.

Read more: How Force changes the State of Motion

### Ship:

Ships can float on the surface of the sea due to the volume of water displaced by the ship is enough to have a weight equal to the weight of the ship. Ships are constructed, having hollow shapes, which make the overall density of a ship lesser than the seawater. This is why the buoyant force acting on the ship is large enough to support its weight.

### Fish:

Just as earlier stated, some certain groups of fishes and swimmers also use the principle of Archimedes’ to move up and down the water. To go up to the water surface, the fishes will fill their swim bladder (air sacs) with gases. These gases diffuse from their own body to the bladder, making their body lighter, which enables the fishes to go up.

Note: the purpose of buoyancy is to keep objects float or sink in water. This is because of the differences in pressure acting on opposite sides of an object immersed in a static fluid.

Read more: Relationship between Force and Motion

## Causes of buoyancy

It is mandatory to know that buoyancy is caused when the pressure exerted by the fluid in which the object is immersed. Also, the buoyant force experienced by the object is always upwards. This is because the pressure of the fluid increases with the depth. Some examples of buoyancy also include:

• A boat or a ship floating in the water
• An immersed cork in water will end up floating due to buoyancy.
• Swimmers are good examples of buoyancy.

## Types of buoyancy

Below are the various types of buoyancy. The three types of buoyancy are positive, negative, and neutral buoyancy.

Positive types of buoyancy occur when the immersed object is lighter than the fluid displaced. This is why the object floats.

Negative buoyancy types are when the immersed object is denser than the fluid displaced, resulting in the object sinking. Finally,

Neutral types of buoyancy occur when the weight of the immersed object is equal to the fluid displaced. Below are the factors that can affect the various types of buoyancy.

• The density of the fluid.
• The volume of the fluid displaced can also affect buoyancy. Lastly,
• The local acceleration due to gravity.

Factors that do not affect buoyant force include;

• The density of the immersed object
• The mass of the immersed object is also a factor.

## Density and relative density

Understanding the concept of buoyancy, one must properly understand the concept of density and relative density.

The density of materials can be defined as their mass per unit volume. It is a measurement of how tightly matter is packed together. It is numerically defined as:

Densit,ρ=MassVolume=MV

• The SI unit of Density is measured using kilogram per cubic meter (kg/m3).
• The density is 0.9584 grams per cubic centimeter at 100° Celsius.

The relative density of a substance is defined as the ratio of the density of the substance to the density of water at C. It is also known as the specific gravity of a substance.

RelativeDensity=DensityofasubstanceDensityofwaterat4∘C

Since relative density is a ratio of similar quantities, it has no unit.

## Why object float or sink in water

The reason objects float or sink in water can be considered when one imagined water consists of overlying layers. That is one over the other with varying pressure. The pressure at the bottom of the liquid is greater than that at the top, which is why we sink in the liquid in some layers. These layers tend to increase as the object sinks.

With the difference in pressure in the fluid layers, a made-up force is being applied in its upward direction. This force result in the acceleration of the object that has been submerged in the upward direction. The force is always in the vertical direction.

In another word, this can also be said that the magnitude of the upward force is basically equivalent to the difference in the pressure of the topmost. The last layer is also equivalent to the weight of the fluid that has been displaced.

The consequence of the above concept is what we called floating. The object should be less dense than the water, else, greater density will result in the sinking of the object.

## Conclusion

Buoyancy or upthrust is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. It is also said to be the force that causes objects to float. it occurs when the differences in pressure acting on opposite sides of an object immersed in a static fluid. That is all for this post where we discussed the definition, applications, purpose, causes, and types of buoyancy.

I hope you get a lot from this post, if so, kindly share with other students. Thanks for reading, see you next time!